About RingSizes

RingSizes is a free tool site for anyone buying a ring — online, second-hand, or as a surprise. It turns one measurement — a finger's circumference, a ring's inside diameter, or a size in any national system — into the equivalent size in every system that matters: US, UK, EU/ISO, French, Italian, German and Japanese.

Where the numbers come from

Every conversion on this site is computed from one published relation, not copied from somebody's chart: the standard US formula (circumference = 2.55 × size + 36.5 mm, diameter = 0.8128 × size + 11.63 mm) as documented for ISO 8653 and the traditional US scale. UK letters follow the BS 6820 letter scale; EU sizes are the inside circumference in millimetres; French and Italian subtract 40; German states the diameter; Japanese and Chinese share the East-Asia numbering. Where national manufacturers disagree by a size, the site says so instead of pretending precision.

What this site is not

It is not a substitute for a jeweller's fitting. Fingers swell and shrink through the day, wide bands fit differently from narrow ones, and a non-resizable purchase deserves a professional sizing first. Treat every result here as the honest starting point the formulas give you — then verify with a physical sizer or a store visit before buying something that cannot be exchanged.

Privacy in one line

All calculations run in your browser. Nothing you type into a calculator is stored, uploaded, or sent anywhere. See the privacy policy for the full story.

Questions or corrections? Contact us — formula checks and chart disagreements are genuinely welcome.